The Count: The Error of Relying on Fielding Percentage

The New York Yankees’ 18-game streak without committing an error, which ended earlier this month, highlighted the stat’s increasing irrelevance to analysis of fielding proficiency.

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Though Ryan Zimmerman has been one of the bright spots for the Nationals’ defense, his dive for this ball (and eventual drop) sums up Washington’s fielding this year.

While the Yankees’ feat was impressive, it also happened in an environment when fewer plays are being scored as errors than ever before. Through Tuesday, the overall major-league fielding percentage this year — the sum of putouts and assists divided by the sum of putouts, assists and errors — is 0.9841, the highest in major-league history.

Fielding proficiency is inching slowly towards perfection, from 0.9837 last year, 0.9836 the year before and 0.9832 in 2006, when the Red Sox set the previous record for an errorless streak of 17 games, according to data provided by Sean Forman of Baseball Reference. Yet such small shifts have a big effect on the probability of a team emerging from a given set of 18 straight games with clean stat sheets. For an average team so far this season, that probability is one in 59,100. That’s minuscule, but it’s twice as likely as it would have been when Boston pulled off its shorter streak. And it’s nearly 11 times more likely than an 18-game streak would have been when the St. Louis Cardinals set the prior record of 16 games in 1992, with a leaguewide fielding proficiency of 0.9811.

Those probabilities are for any given 18-game stretch. Once you take into account all such stretches played by all 30 teams, probabilities are much higher. There was a 2.9% chance of a streak that long so far this season, and a 6.7% chance for all of last season. Expansion of the season and the major leagues has also increased the probability of an errorless streak. A half-century ago, when 16 teams playing 154 games per season had an average fielding percentage of 0.9767, the probability of an 18-game errorless streak was just one in 5,770. A century ago, that was one in 125 billion. Again, this is assuming all teams have the same fielding percentage, with the same number and difficulty of chances in each game. Accounting for good-fielding teams hitting lucky streaks would increase the probability.

Another shift has also tilted the odds in favor of errorless streaks: Fielding units are getting fewer chances per game. The average chances per team per game this season are 37.41, down from 37.55 in 2006, 38.44 in 1992 and rates of 40 or above in 1928 and all years earlier. That could reflect a long-term rise in strikeouts and home runs.

So we can expect more errorless streaks. And when they happen, we should look more deeply at teams’ overall fielding performance. Shortly after the streak ended, Matthew Carruth pointed out on FanGraphs that the Yankees’ defense ranked 14th overall in Ultimate Zone Rating, which takes into account the range of fielders in addition to their errors — “which isn’t much to write home about.” There are signs that the Yankees really were fielding well during their streak, but since then their ranking in UZR has fallen to a tie for 22st out of 30 teams — though they rank 9th in fielding percentage.

Spectacular plays also aren’t a particularly reliable way to measure team defense. Blogger Nick Kapur noted that the Washington Nationals top the leaderboard for “Web Gems,” the title assigned by ESPN’s “Baseball Tonight” program for the best plays of the day, yet rank last according to UZR — far behind the Yankees. (The Nationals also rank last in fielding percentage, suggesting they have poor range and misplay the ball unusually often when they get to it.)

“I can only presume that the Nats players have to dive so much because they reach the edge of their range so quickly,” Kapur wrote. An ESPN spokesman told me, “Web Gems are associated with spectacular individual plays, not the competitive level of a team, not team defensive efficiency, and not any kind of game outcomes.”

UZR isn’t perfect. For instance, it can suggest that a player is improving or declining when really it’s his peers who are changing. But it’s a lot better than fielding percentage or the naked eye.

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